Traveling wave devices



United States Patent 4 TRAVELING WAVE DEVICES Robert McCowan Unger, Wayland, Mass., assiguor to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware Application April 25, 1956, Serial No. 580,609

9 Claims. (Cl. S15-3.5)

This invention relates to a traveling wave electron device and particularly to means for matching the impedance of the periodic slow wave propagating structure of a traveling wave electron discharge device to the impedance of an output coupling means used for removing energy from said device.

Traveling wave oscillators are known in which the electrons in an extended beam are caused to interact with the electromagnetic eld of a wave propogating along the path adjacent the periodic slow wave energy propagating structure, which may be an interdigital delay structure. The electromagnetic field along such a structure may be considered to consist of a number of superimposed traveling waves or space harmonics. These harmonics travel with different phase velocities, some of which are characterized in that the phase velocity lis in the same direction as the energy or group velocity; such harmonics are rel fferred to as forward waves. Other harmonics, on the other hand, are characterized by a negative phase velocity, that is, a phase velocity in a direction opposite to the energy or group velocity; these waves are referred to as negative space harmonics or backward waves. If, for example, the electron beam is propagated in the direction of one of these backward waves .at a'velocity substantially equal to the phase velocity of the backward waves, interactionl will occur between the electron beam and the traveling wave and the energy will be transferred to the electromagnetic eld; the energy given to the backward wave will be transferred along the periodic structure toward the beam source. If the electron beam exceeds a critical value at which oscillations can begin, and when the electron beam velocity is in substantial synchronism with the velocity of one of the space harmonics, such as a backward wave, oscillations may be generated in the tube. These oscillations willpropagate along the periodic structure and may be extracted at one end thereof by means of an l output coupling means, such as a coaxial line, whose inner conductor is attached to the periodic structure adjacent one end.

In one type of traveling wave tube the electron beam is permitted to pass through the interaction space adjacent the periodic slow wave propagating structure and may be collected at the end thereof remote from the electron source by a collector electrode maintained at a potential positive relative to the electron source. The electron beam is prevented from moving laterally within the tube by a longitudinal focusing magnetic field. In other types of traveling wave tubes the electrons are permitted to move through unidirectional electric and magnetic fields which are transverse to one another and to the mean path of the electron beam.

Interdigital transmission networks have been found to be particularly suitable for use as the periodic slow wave propagating structure of traveling wave tubes. Such interdigital networks comprise a pair of arrays each including a continuous portion or base from which depend several spaced elements or lingers, the fingers of one array extending into the spaces between adjacent `fingers of the other "ice array. The characteristic impedance of an interdigital line is equal approximately to the product of a constant (377 if air is used as a dielectric) and the ratio of the space between adjacent fingers to the width of said fingers. A satisfactory output coupling means comprises a coaxial line having an inner conductor and a concentric outer conductor. In the case of backward wave traveling wave oscillators, the output coupling means it attached to the interdigital line adjacent the upstream end, that is, at one end thereof adjacent to the electron source. Specifically, the inner conductor of the coaxial line may be mechanically and electrically connected to the end finger of the interdigital line while the outerl conductor is electrically connected to the base of either array of the interdigital structure.

In order to prevent undesirable reflections of energy from the end of the interdigital structure, that is,'\from the juncture of the interdigital structure and the coaxial line, it is essential that the impedance of the interdigital structure be matched to that of the coaxial line. It has been found in practice that the'characteristic impedance of interdigital networks suitable for use in traveling wave tubes differs from the characteristic impedance of the coaxial transmission line. Some means, therefore, is required to provide the necessary adjustment for obtaining the optimum impedance match. If the end finger of the interdigital structure of the tube is sufficiently flexible, the position of this finger, otherwise referred to as a matching linger, may be adjusted to provide the proper impedance match between the interdigital structure andthe coaxial line. This adjustment is often necessary in order to compensate for variations from tube to tube. If the position of the matching finger is adjusted at cold test, that is, While the tube is operating under other than actual operating conditions desired in practice, the spacing will usually differ appreciably when the tube is actually used in some particular application. This difference results, in part, from differences in thermal expansion of the materia'ls of the lines and from thermal gradients which cannot be predicted in advance of actual operation. It is desirable, therefore, to adjust the position of the matching finger of the interdigital line until the proper impedance is attained during actual operation without breaking the vacuum seal.

A sliding joint is provided on the outer conductor of the coaxial output coupling means and a Vacuum seal is retained by means of a resilient diaphragm connected to the movable and stationary portions of the outer conductor of the coaxial output coupling means. The outer conductor may include a part of the base of the interdigital network, that is, the portion of the network to which the lingers are attached. The end of the inner conductor not attached to the matching linger is aixed to the movable portion of the outer conductor through a dielectric vacuum seal. Since the length of the inner conductor is fixed, movement of the movable portion of the outer conductor will cause axial movement of the matching linger and will permit the proper impedance match to be obtained between the coaxial line and the interdigital network. Once the optimum adjustment has been obtained, as determined by measurements indicating optimum output characteristics, the sliding joint may be fixed in position. Y

For a better understanding of the invention, together with further objects thereof, reference is made to the following description taken in conjunction with the accompanying drawing wherein:

Fig., l is a view, partly in section, illustrating an embodiment of the invention;

Fig. 2 is a plan view of the accelerating anode used in the electron gun assembly of Fig. 1;

Fi g 3 isa fragmentary view showinga portion of the nterdigital delay network of Fig. 1;

Fig. 4 is an exploded view of a portion of a laminated nterdigital network which-maybe used in lieuof the solid interdigital networkshow-n in Figs. l and 3; and

N Fig'. 5 :is a View showing a-portionof the coaxial output meansoFig. 1.

Y"Referring tothe drawing, a traveling wave-tube 10 is -shownwhich includes-an electron gun assembly 1.2 mounted within a cylindrical housing 13, an nterdigital slow wavepropagating structure 40, a focusing magnet 55. and a coaxial output assembly 60. Details of the traveling `Wave tube, using a laminated nterdigital structure, are ff'ully d'sclosed in a Ycopending application by Rudolf C. Hergenrothen -Serial Number 579,972, filed April 23, .1956.

The .electron Vgun assembly comprisesa cathode 22 including therein a heater coil 23, a grid 24, an accelerating v4anode 25, and mounting plates 26 and 27, the latter of which lis secured-to one end of ,housing 13. The elements 2 2 to 25 of the electron gun are insulatedly mounted in spaced v`relationship by means of ceramic support rods 23 v vhich pass through mounting plates 26 and 27 and element l25. A ceramic-to-metal seal may be made at the points of .insertion of the support rods 28 into-the accelerating anode 25 and themounting plates 26 and 27. The grid 24 is supported from mounting plate 25 by means of yone or more wires 29 spot welded to the grid and extending through mounting plate 26. A glass bead 31 is attached to one end Iof wires 29, while wire 32 is secured to theglass beads 31 as shown in Fig. l. The cathode 22 is lstlpporteddrom mounting plate 26 by means of a support Wire 33 spot Welded to the cathode and passing through Va central aperture in mounting plate 26. This support wire 33 vfor the cathode is attached to wire 32.

Grid lead 34 is secured directly to the mounting plateY l 2 6, lwhile a cathode lead 35 is connected to wire 32. A heater wire 36 interconnects one end of heater 23 and oneof the wires 32'; a heater lead 37 is attached tothis Wire 3 2'. The other end of the heater 23 may be connected directly to the cathode. The accelerating anc-de 25 --is mounted on support rod 28 in spaced relationship with vthe grid 24, and a lead 38 is connected to the accelerating anode V25. The housing 1 3 is maintained yat the-same potentialras the nterdigital network 40 and an 4appropriate source of high voltage, not shown, is connected between the network 49 and cathode 22. Likewise,

appropriate sources of potential, not shown, for the other elements of lthe electron gun are necessarily provided. flfhe leads 34, 35, 37, and 3 8 extend through a seal, not shown, mounted lat the end of housing 13 remote from mounting -plate 27. The grid, accelerating anode and mounting plate 27 contain two parallel slots through `which electrons from the cathode may pass. A plan view of the accelerating anode 25 containing slots 39 is shown in Fig. 2. By means of these slots, and by means of appropriate accelerating voltage between the accelerating anodee25 and the cathode 22, a pair of substantially fiat beams is directed into the interaction space 45 of the nterdigital delay network 40 adjacent the edges of the ngers 44. A portion of the interaction space 45 is visi-ble in Fig. 3.

The axial focusing eld for the electron beam is pro vided by the magnet assembly 55 which is mounted about the nterdigital delay structure 40 -by means yof a ring plate assembly 56 positioned at each end of the tube. Set screws 57 passing through a portion of each assembly 56 and seating Iagainst electron gun housing 13 and the nterdigital delay network d0, respectively, maintain the axial magnetic eld producing means 55 in the desired relationship relative to the nterdigital delay network- 40.

As shown in Figs. 1 and 3, the nterdigital delay network 40 may be attached at one end to mounting plate 27 which in turn is aiiixed tohousing 13; the nterdigital network 40 may consist of two solid assemblies 41 and 42 joined together as shown in Fig. 3. The upper assembly 41 includes a body portion d3 from which extends a plurality of spaced iingers 44. The lower assembly 42 includes a body portion 46 from which extends a plurality of iingers 44 which protrude into the spaces between adjacent ngers 44 of the upper Vassembly 41. A circular recess or `bore 48 is provided in the body portion 46 of the lower assembly 42 and the periphery thereof serves as a portion of the outer conductor of the coaxial output coupling means 6i), The inner conductor'l, which includes a reduced portion 61a and alarger portion 6th, is disposed centrally withinthe recess 43 and one end thereof is attached to the end of matching finger 44' of the upper assembly 41. The matching finger 1A is longer than the remaining fingers dal'ofthe upper assembly 41 so that connection may be made directly to the reduced portion 61a of inner conductor 61. The reduced portion 61a of the inner conductor 61, together with the portion .of the outer conductor surrounding it, serve as a quarter wave section having a characteristic impedance equal vto the square root of the product of the characteristic impedance 'of the nterdigital network il and thc'characteristic impedance of that portion of the coaxial line including the enlarged portion 6l!) of the inner conductorjl. ln this manner, the impedance of the nterdigital network may be matched to the impedance of the output line, which normally is different than that of the nterdigital network. Although such a quarter wave transformer 4is frequency sensitive, substantial matching over a considerable `range may be provided by virtue of the adjustable matching finger 44.

Alternatively, the nterdigital network may be made up of several laminations, as shown in Fig. 4. .The major portion of the nterdigital line then .includes a plurality of sets of three laminations S0, 51, and 52, each containing a circular aperture 14S near one Vend, whase boundary serves as a portion `of the outer conductor of the .coaxial output coupling means dit when the laminations have been stacked together and assembled, as by brazlng. Each Vlamination includes a generally Arectangular opening 53. Laminations Sil include an'elongated electrically conductive Yfinger 144 which extends from one edge of opening 53almost to the opposite ,edge thereof. Laminations 52 likewise include an electrically conductive nger 144 which extends from the edge of opening 53 opposite that of lamination Si) toa point adjacent, ybut not contacting, `the opposite edge ofvsaidopening. The elements or iingers 144 of laminations 52, when assembled, are positioned in vspace relationship with `adjacent ngers of laminations 50 .by means of .spacer laminations 51 which do not contain any. fingers. The matching nger 144 of laminationSZ is similar to `that of lamination 52 except that the finger 144 is slightly longer in order to permit connection to be made directly to'theinner conductor '61. The opening 53 in lamination 52', unlike the openings S3 in the other laminations, is continuous with the circular aperture forming the boundary of the outer conductor of that portion of thecoaxial output means'tl disposed within the nterdigital network 40. Spacer 51 is similar to that of spacer lamination 51, except that it con tains an enlarged opening 53', like that in lamination 52', which may be provided at the end of the nterdigital network 50 in order to allow room for sucient movement of the matching finger 144'.

As indicated in Fig. 5, the xed portionof the outer conductor of the `output coupling means 60 includes a hollow 4member 63 vhaving a reduced portion 64 of inner diameter equal to that of the recess 48, in the case of the so-lid nterdigital construction of Figs. 1 and 3, or equal to that of the circular apertures .148, in the case vof the laminated nterdigital construction of `Fig. 4. The end of the reduced portion is secured, as by brazing,.to.the end of the lower assembly 42 of Fig. 3, or into one or. more of the-apertures 148 in the laminated assembly of Fig.- 4.

Thev enlarged portion 65 of member 63 terminates in an end face 66. The movable portion of the outer conductor of the coaxial output coupling ymeans 60v includes an output supporting element 68 through which the inner conductor 61 extends. The inner conductor 61 is mounted centrally within element 68 by means of a dielectric spacer disc 69 which may be made of ceramic. Since the dielectric constant of the ceramic disc 69 differs from that of air, the impedance of the coaxial line is maintained constant by means of a conductor 71 of reduced diameter which passes through a central aperture in the ceramic disc 69 and its into recesses in the inner conductor 61 and the inner conductor 161 of a coaxial connector 160 which also includes an outer member 162. An output sleeve 72 whose outer periphery is threaded, as indicated in Figs. l and 5, is secured to the supporting element 68, as by a solder ring positioned at 73. The outer member 162 of coaxial connector 160 is secured to one end of the output sleeve 72. The ceramic disc 69 is secured to the inner conductor assembly 61b, 71, 161 as by insertion of solder rings, not shown, which may be positioned about conductor 71 and which, after application of heat, flow into the aforementioned recesses. The ceramic disc 69 is attached to the supporting element 68 by means of solder rings placed at 7S. A llexible diaphragm '76 is connected between one end of the supportingV element 68 and the enlarged portion 65 of member 63. An adjusting nut 77 having a peripheral ange 78 threadedly engages the periphery of output sleeve 72. In order to adjust the position of the matching nger 44' (or 144') of theinterdigital delay network 40, the nut 77 is rotated about the threaded output sleeve 72 until the face 79 of the peripheral flange 78 bears against the .face 66 of member 63.' Since member 63 is xed inposition, further rotation of the nut 77 causes the output 4sleeve 72 to move toward the right in the drawing, and the inner .conductor assembly, including element 61 which is .attached to the output sleeve 72 through the ceramic disc 69 and supporting element 68, also moves with the output sleeve. The movement of the inner conductor61 consequently produces a physical displacement of the ilexible matching finger 44 (or 144') attached to the far end of the inner conductor 61. As the inner conductor 61 is moved axially as a consequence of rotation of the adjusting nut 77, the ilexible matching finger 44 is bent toward oraway from its neutral position, thatis, parallel tothe adjacent linger of the interdigital delay network 40, -by means of cantilever action where the fulcrum is the point of attachment of the nger 4d' to the body portion 43 of the interdigital network 46. By varying the spacing between the flexible matching finger 44' and the adjacent finger of the interdigital network 40, the matching impedance may be altered over a considerable range, thereby compensating for variations in impedance of the network 46 owing to slight distortion of the network, such as bent fingers, and for variations in the characteristics of the quarter-wave transformer, previously referred to, with frequency changes. Furthermore, some compensation may be made for slight deviation of the inner conductor of the output coupling means from the center of the coaxial line, and for slight discontinuities in the coaxial output coupling means. In practice, the nut 7'7 is backed off along the threaded output sleeve 72 and the assembly, including the inner conductor 61, is

manually pushed toward the left, as shown in the drawing, as far as possible; the nut is then rotated clockwise until contact is made between the faces 66 and 79 of the member 63 and nut 77, respectively. Once the proper adjustment of the matching linger has been attained, nut

77 may be brazed in position as at points 81 and 82.

This invention is not limited to the particular details of construction, materials and processes described, as

' many equivalents will suggest themselves to those skilled in the art. For example, the supporting element 68 may be omitted in certain instances and the inner conductor `61 may be iixedly supported directly within the output sleeve 72 by means of an insulating disc 69 whose .outer periphery engages the inner periphery of the output sleeve 72. In this event, the diaphragm 76 could be connected to the output sleeve 72, rather than to the supporting element 68, as shown in Fig. 5. ln most practical applications, however, the method shown in Fig. 5 is preferred by reason of mechanical considerations. It is, accordingly, desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. In combination, a periodic slow wave energy propagating network including a plurality of interdigital spaced elements, and coaxial output coupling means coupled to said network, said output coupling means including means for adjusting the position of one of said elements of said network, said output coupling means including a stationary portion and a movable portion rigidly `connected to said one element, and means for adjusting the position of said one element in response to relative movement of said movable and stationary portions, said output coupling means including a exible hermetic seal between said stationary and said movable portions.

2. In combination, a periodic slow wave energy propagating network including a plurality of spaced elements, and coaxial output coupling means coupled to said network, said output coupling means including a stationary portion and a movable portion rigidly connected to one only of said elements, said movable portion including an inner conductor, a tubular member whose outer periphery is threaded, and means for xedly supporting said inner conductor relative to said tubular member in hermetically sealed relationship, flexible means interconnecting said movable portion and said stationary portion and providing a hermetic seal therebetween, and a nut threadedly engaging said member for providing movement of said one element only of said network in response to a force exerted by said nut against said stationary portion of said output coupling means.

3. A traveling wave tube comprising an evacuated envelope containing therein means for directing a beamof electrons along an extended path and means for propagating electromagnetic wave energy adjacent said path at a velocity substantially equal to that of said electron beam, said means for propagating including a plurality of spaced elements, an output coupling means for removing-energy from said means for propagating, said output coupling means including a stationary member and further including a movable member rigidly connected to a rst portion of said means for propagating, and means for adjusting the position of said portion of said means for propagating relative to the remainder of said means for propagating said output coupling means being hermetically sealed to the tube envelope during adjustment of the position of said rst portion of said means for propagating.

4. A traveling wave tube comprising means for directing a beam of electrons along an extended path, means for propagating electromagnetic wave energy adjacent said path at a velocity substantially equal to that of said electron beam, said means for propagating including a plurality of spaced elements, an output coupling means for removing energy from said means for propagating, said output coupling means including a stationary portion and including a movable portion rigidly connected to one only of said elements, and means for adjusting the position of said one element only in response to relative movement of said movable and/ stationary portions said output coupling means being hermetically sealed to the tube envelope during adjustment of the position of said one element.

5. A traveling wave tube comprising an evacuated envelope containing therein means for directing a beam of electrons along an extended path and means for propagat-` ing electromagnetic wave energy adjacent said path at a velocity substantial-ly equal `to that of said electron beam,

saidfmeans"forgpropagating including afplurality of: speed elements, -=an output coupling means for removing-energy 'fromwsaid'means for propagating, said output coupling means including'av stationary portion and further including a movable portion rigidly connected -to one of said elcments, and means for adjustingthe position of said one element in response to relative movement of said movable andl stationary portions, said output couplingmeans being hermetically sealed to the tube envelope during adjustment of the position of said one element.

6. A traveling Wave tube comprising means for directing'a beam of electrons along an extended path, means for propagating electromagnetic Wave energy adjacent said -path at a velocity substantially equal to that of said electron beam, said means 'for propagating including a plui rality of spaced elements,` output coupling means for removingenergy from said means for propagating, said output coupling means including an outer conductor and an inner conductor, said inner conductor being rigidly connected to one ofsaid elements, said outer conductor being isolated from and coaxial with said inner conductor, said outer conductor including a movable portion-rigidly attached to said inner conductor and further including a stationary portion, means for effecting a flexible hermetic seal between said stationary land said movable portions, and means for adjusting the position of said one element in response to relative movement of said movable and stationary portions.

"7. A traveling-Wave tube comprising means for directing la beam of electrons along an extended path, means for propagating electromagnetic wave energy adjacent said path at a velocity substantially equa-l to that of said electron beam, said means for propagating including -a plurality of spaced elements, output coupling means 'for removing energy from said means for propagating, saidoutl put coupling means including an outer conductor and an inner conductor, said inner conductor being rigid connected to one of said elements, said outer conductor being isolated from and coaxial with said inner conductor, said outer conductor including a movable portion rigidly atta-ched to said inner conductor and further including a `stationary portion, means for effecting a flexible hermetic seal =between saidv stationary and said movable portions, meansfor providing a lixed hermetic seal between said inner 'conductor and said movable portion of said outer conductor, and `means for adjusting the position ol said '18 one element-in response to relative movement of said movf ablefandlstationaryportions.

:8. A traveling Wave `tubecomprising means for directing-a Ibeam of electrons along an extended path, means 'forpropagating electromagnetic wave energy adjacent said path at a velocity substantially equal to that of said electron-beam, said-means for propagating including a plurality of spaced elements, Vcoaxial output coupling means coupledto said network adjacent one end thereof, said output coupling means including a stationary portion and a movable portion, said movable portion including an inner conductor rigidly connected to one of said elements, a tubular member whose outer periphery is threaded, means for iixedly supporting said inner conductor relative to said Itubular member in hermetically sealed relationship, flexible means interconnecting said movable portion and said stationary portion and providing a hermetic seal therebetween, and a nutthreadedly engaging said member for providing movement of said one element of said net- Work in response to a force exerted by said nut against said stationary portion of said output coupling.

9. A traveling Wave tube-comprising means for directing a beam of electrons along an extended path, means for propagating electromagnetic Wave energy adjacent said path at a velocitysubstantially equal to that of said electron beam, said means for propagating including a plurality of spaced elements, said output coupling means including a stationary portion and 'a movable portion, said movableportion including an inner conductor rigidly connected-to one of said elements, a tubular member, means for xedly supporting said inner conductor within said tubular member in hermetically sealed relationship, exible means interconnecting said tubular member and said lixed portion and providing a hermetic seal therebetween, a threaded sleeve axed to said tubular member, and a nut threadedly engaging lsaid sleeve for providing movement of said movable portion in response to a force exerted by said nut against said stationary portion of said output coupling.

References Cited in the lle of this patent UNITED STATES PATENTS "2,622,158 Ludi Dec. 16, 1952 2,659,032 Crawford Nov. 10, 1953 2,765,421 Robertson et al Oct. 2, 1956 ,f wfff UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No., 2,837,684 June 3, 1958 Robert McGowan Unger It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters- Patent should read as corrected below.

Column 2, line 89 for "it attached" read. vmis attaoldechmj,7 column line 52, after "propagating" insert a comme; line 67, after "portions insert a comma; column '7'y line l9 for "speed" read .mspaeemg line 3'7, for "rigid" read -=-rigidly=== Signed and sealed this 29th day of July 1958.,

(SEAL) Attest: KAEL E, AXLTNE ROBERT C. WATSGN Attesting Oficer Commissioner of Patents 

